1. Field of the Invention
The present invention relates to an image forming method using a two component developing system in which a two component developer having a toner and a carrier is carried on a developer carrying member, and a developing bias including a DC voltage and an AC voltage superimposed on the DC voltage is applied to the developer carrying member to develop the toner on an electrostatic image formed on an image bearing member.
2. Description of the Related Art
In the related art, in image forming apparatuses such as copying machines and printers using electrophotography, an image bearing member having a photosensitive layer as a surface layer is subjected to charging and exposure to form an electrostatic image, the photosensitive layer being formed with a photoconductor such as an OPC (organic photo conductive) photosensitive member and an amorphous silicon photosensitive member. Thereafter, by a development field caused by an action of the developing bias applied to the developer carrying member, in a development region in which the image bearing member faces a developer carrying member, the electrostatic image is developed by a toner to form a toner image on the photosensitive member. Further, the toner image on the photosensitive member is transferred on a transfer material directly or via an intermediate transfer member. Subsequently, the toner image is fixed on a transfer material such as paper to obtain a recorded image. Particularly, in the image forming method using the two component developing system, when the two component developer including at least a toner and a magnetic carrier is conveyed to the development region by the developer carrying member, the toner is separated from a magnetic carrier particle by the development field generated by the developing bias, and as a result, the electrostatic image formed on the photosensitive member is electrostatically developed.
Recently, higher print speed and higher quality of an output image have been demanded for the copying machines and printers, and at the same time, reduction in environmental load in the print process has been strongly demanded. For example, as a technique for reducing power consumption during printing, a technique has been developed in which the toner is fixed on the transfer material by reducing the melting point of the toner to lower the fixing temperature. If the melting point of the toner is reduced, however, the temperature is raised by stirring the developer or the viscosity of the toner is increased by change in an environment or the like, leading to a non-electrostatic adhesive force between the magnetic carrier and the toner. For this reason, developability is undesirably reduced over time in printing for a long period of time.
In the related art, in the image forming method using the two component developing system, an alternating bias including a DC voltage and an AC voltage superimposed on the DC voltage is used as the developing bias. Thereby, the amount of the toner to be developed can be increased, and a recorded image having high density and high quality can be output.
Unfortunately, at a higher process speed of not less than 300 mm/s, it is found out that the time during which the developer passes through the development region is shortened, therefore reducing the time during which the toner is exposed to the development field, leading to difficulties to keep a sufficient amount of the toner to be developed. It is also found out that if the peak-to-peak voltage of the AC component in the developing bias is increased in order to ensure a sufficient amount of the toner to be developed, and the developer proposed in the related art is used, a peak-to-peak voltage of not less than 1.5 kV is needed to output a recorded image having a desired density.
Unfortunately, it is found out that if the peak-to-peak voltage of the AC component in the developing bias is more than 1.3 kV, the magnetic carrier adheres onto the photosensitive member to cause a phenomenon in which the remains of the magnetic carrier on the toner image manifest themselves as a blank. The reason is thought as follows. Increase in the process speed leads to a higher conveying speed of the developer in the development region, and a centrifugal force of the magnetic carrier separating from the developer carrying member is also increased. Moreover, increase in the peak-to-peak voltage of the developing bias leads to increase in an amount of charges having the same polarity as that of the toner to be injected into the magnetic carrier. As a result, the magnetic carrier easily moves from the developer carrying member to the photosensitive member by the development field.
For this reason, in order to reduce the carrier remains in the image forming apparatus having a process speed of not less than 300 mm/s, development of an image forming method has been desired in which a recorded image having a desired density can be output while a peak-to-peak voltage Vpp of the AC component in the developing bias is not more than 1.3 kV.
In the image forming method using the two component developing system, electrical properties of the magnetic carrier have a great influence on the local electric field applied to the magnetic carrier particles and the electrostatically adhering toner particles. Accordingly, in the two component developing system, the developability depends on the electrical properties of the magnetic carrier. In the related art, attempts have been made to improve the developability by adjusting the electrical properties of the magnetic carrier based on such a phenomenon. Particularly recently, in order to avoid irregularities in the latent image caused by reduction in the resistance of the magnetic carrier, a method has been proposed in which the permittivity of the magnetic carrier is increased to improve the developability while the image quality is kept.
For example, a method has been proposed in which the magnetic carrier contains a permittivity material; thereby, the developability is improved and a desired image density is ensured while the electric resistance of the magnetic carrier is kept high to reduce the charges to be injected into the electrostatic image.
Japanese Patent Application Laid-Open No. S60-19157 and Japanese Patent Application Laid-Open No. H10-83120 propose a magnetic carrier coated with a high resistance substance, wherein the high resistance substance contains a high permittivity substance; thereby, high reproductivity of a high density portion and a halftone is provided while the electric resistance of the magnetic carrier is kept high. Unfortunately, in the method for dispersing a permittivity material in a high resistance coating material, if printing is performed for a certain period of time or longer, wear of the coating layer reduces the effect of the permittivity material. For this reason, the developability is reduced, leading to reduction in the image density and poor granularity in an output image. Moreover, coating of the surface of the magnetic carrier with a high resistance substance inhibits movement of the charges between the magnetic carriers. For this reason, charges having an opposite polarity to that of the toner may be accumulated within the magnetic carrier during development of the toner, and the magnetic carrier may adhere onto a blank area of the photosensitive member, causing image defects.
Moreover, Japanese Patent Application Laid-Open No. 2007-102052 proposes a magnetic body dispersing type resin carrier having a magnetic particle dispersed in a resin, wherein a high resistance substance having a relative permittivity of not less than 80 is dispersed in a binder resin; thereby, an image having a stable density can be output for a long period of time while the resistance of the magnetic carrier is kept high. Unfortunately, in the method for producing a magnetic carrier core by dispersing the magnetic material and the permittivity material in the binder resin, the amount of the magnetic particle to be dispersed in the binder resin is limited, and the amount of the magnetic carrier to be magnetized cannot be increased. If the process speed is higher, problems occur, i.e., the transportability of the developer is reduced, or part of the magnetic carrier adheres onto the photosensitive member, causing image defects such as the remains of the magnetic carrier appearing on the image.
Moreover, the high permittivity material used for the magnetic carrier above is more expensive than the magnetic materials and resin materials used in the magnetic carrier in the related art. The problem of production cost is left unsolved in use of the high permittivity material having high quality in order to obtain the effect of permittivity.
Another method for improving developability has been proposed in which without using a permittivity material, an electrically conductive path in the magnetic carrier under the development field is controlled to increase the effective permittivity.
For example, Japanese Patent Application Laid-Open No. 2010-170106 proposes a method for improving developability in which in a resin-filled type ferrite magnetic carrier obtained by filling pores of a porous ferrite particle with a resin, a state of the contact between the porous ferrite components in the ferrite particle is varied to control the electrically conductive path of the magnetic carrier and increase the permittivity substantially. Unfortunately, in the method for varying an inner contact state of porous ferrite particles at a certain level or more, which is proposed in Japanese Patent Application Laid-Open No. 2010-170106, even particle diameter distribution needs to be managed, and it is difficult to keep production stability and to produce a magnetic carrier having stable properties. Further, a raw material ferrite having different center particle diameters or particle diameter distribution needs to be produced, leading to a complicated production process. Accordingly, the method is not suitable for reduction in production cost.
Japanese Patent Application Laid-Open No. 2007-218955 proposes a magnetic core having micropores inside thereof, the magnetic core having a magnetic phase as ferrite and a non-magnetic phase containing one or more of SiO2, Al2O3, and Al(OH)3 as a unit for increase the resistance of the magnetic core. By use of the magnetic core having a magnetic phase as ferrite and a compound having a non-magnetic phase, keeping the resistance of the magnetic carrier high is improved, and reduction in the image quality by the charge injection is prevented. Unfortunately, the structure having a non-magnetic phase obstructs increase in mass susceptibility of the magnetic carrier. For this reason, if the process speed is higher, part of the magnetic carrier adheres onto the photosensitive member, causing image defects such as the remains of the magnetic carrier appearing on the image.
As above, the methods proposed in the related art do not sufficiently solve the various problems. Accordingly, an image forming method has been desired in which the electrostatic latent image bearing member has a surface circumferential speed (process speed) of not less than 300 mm/s, the peak-to-peak voltage of the AC component in the developing bias is not more than 1.3 kV, and a high density recorded image without carrier remains can be output.